A fuel cell includes an anode (that is, a fuel electrode) to electrochemically oxidize supplied fuel, a cathode (that is, an air electrode) to electrochemically reduce an oxidizing agent, and an electrolyte membrane that is interposed between the anode and the cathode to provide a path for transferring ions generated at the anode to the cathode. Electrons may be generated through a fuel oxidation reaction at the anode, may work via an arbitrary external circuit, and may be returned to the cathode to reduce the oxidizing agent. Since catalysts that are contained in the anode and the cathode and that catalyze an electrochemical reaction are regarded to be very important in the fuel cell configured as described above, various attempts have been made to increase an activity of a catalyst used in an electrode. In general, since an intrinsic activity of a catalyst increases as a reaction surface area of the catalyst increases, an effort has been made to increase the reaction surface area by reducing a diameter of particles of the catalyst, and various attempts have been made to uniformly distribute a catalyst with a higher activity on an electrode to efficiently transfer materials during an inflow of a reactant and a discharge of a product. Since a relatively large number of micropores are provided when a support with a high specific surface area is currently prepared in a catalyst for a fuel cell, it may be difficult to form a triple phase boundary based on conditions required for an oxygen reduction reaction (ORR), and a performance of the full cell may continue to decrease due to a mass transport limitation despite a low loss of a chemical reaction rate. Thus, research has been actively conducted on carbon materials with a large amount of graphite having mesopores. The conditions required for the ORR may include a carbon support, a catalyst of a noble metal, for example, platinum (Pt), and an ionomer, for example, Nafion, that moves a hydrogen cation.
A catalyst support for a fuel cell needs to have a large surface area due to high porosity and a high electrical conductivity for a flow of electrons. Amorphous microporous carbon powders known as activated carbon or carbon black are widely used as a catalyst support for the fuel cell, however, are vulnerable to durability in an operating condition of the fuel cell in a strong acid atmosphere. A porosity and an average pore size that are physical properties of a catalyst layer of an electrode for a fuel cell are key manufacture variables that have a decisive influence on a performance of the fuel cell. The performance of the fuel cell may decrease because water generated in an air electrode, that is, a cathode is not efficiently removed and oxygen required for the ORR is not sufficiently supplied. To increase a performance of the ORR in the air electrode, an attempt was made to use vertically aligned carbon nanotubes (VACNTs) as a catalyst support. However, to grow carbon nanotubes (CNTs) and to support a catalyst on the CNTs, expensive equipment, for example, a plasma-enhanced chemical vapor deposition (PECVD) machine or a sputter, needs to be used in a vacuum condition.
Based on a requirement for a high capacity of a secondary battery, research has been actively conducted on high-capacity anode materials, for example, Tin (Sn) or silicon (Si), having a high capacity per unit weight in comparison to carbon-based anode materials that are widely used. However, when high-capacity anode materials are used, a volume may be expanded, a performance of a battery charging and discharging cycle may decrease.
When crystalline carbon, for example, CNTs or graphene, are used as anode and cathode conductive materials, an electrical conductivity and energy density are very excellent and reversibility of a charging and discharging process is superior to amorphous carbon. There is an industrial demand for a more efficient method of using replacements for anode and cathode conductive materials in which carbon materials, for example, activated carbon or carbon black, are mainly used.
The information included in this Background section of the specification, including any references cited herein and any description or discussion thereof, is included for technical reference purposes only and is not to be regarded subject matter by which the scope of the invention as defined in the claims is to be bound.